What is HLG? The HDR format made for broadcast TV
By TechCompare · Updated · How we check
HLG has no metadata. The bottom of its curve follows the SDR gamma curve and the top switches to a log curve for highlights, so an SDR screen shows a normal picture and an HDR screen stretches the highlights to its own peak.
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HDR10
- What the logo tells you
- Nothing about quality. It's the base signal nearly every HDR screen accepts, so a cheap monitor and a top TV both list it.
- Metadata
- Static: one set for the whole video (SMPTE ST 2086 mastering data, plus MaxCLL and MaxFALL).
- Signal curve
- PQ (SMPTE ST 2084). Each code value stands for a set brightness in nits.
- Bit depth
- 10 bits
- Color primaries
- BT.2020
- Without support
- Not backward compatible with SDR. An SDR screen needs the player to convert it.
- License
- Free to use.
- Who made it
- Announced by the Consumer Technology Association (then the CEA) in August 2015.
- What the screen still needs
- Enough brightness and contrast to show the range the video was mastered for. The screen tone maps whatever it can't reach.
Source: Wikipedia, HDR10
HLG
- What the logo tells you
- The screen can decode HLG, which matters most for live TV and some cameras.
- Metadata
- None. The screen scales the signal to its own peak.
- Signal curve
- Hybrid log-gamma. The low part follows the SDR gamma curve, and the top part is logarithmic.
- Bit depth
- 10 bits (BT.2100 allows 12 too)
- Color primaries
- BT.2020
- Without support
- An SDR screen can show it as a normal picture, though BT.2020 colors can look off on a BT.709 screen.
- License
- Free to use.
- Who made it
- Made by the BBC and NHK for broadcast TV. Part of ITU-R BT.2100 and ARIB STD-B67.
- What the screen still needs
- Brightness and contrast to spare above SDR, or HLG looks like plain SDR.
Source: Wikipedia, HLG
HLG stands for hybrid log gamma. The BBC and NHK made it for broadcast TV, where a channel can't send one stream for HDR screens and another for SDR ones. HLG is one signal that works on both. It's part of ITU-R BT.2100, next to PQ, and it's also in ARIB STD-B67.
How this is calculated
The name says how it works. The lower part of the curve is a gamma curve, close to what SDR uses, so an SDR screen reads it as a normal picture. The upper part is logarithmic, which packs highlights into the top of the signal. An HDR screen knows to stretch that part out.
That's why HLG needs no metadata. PQ, the curve HDR10 and Dolby Vision use, says how many nits each value means, so a screen that can't reach them needs notes on how to tone map. HLG is relative instead. The screen scales the whole signal to its own peak, the way SDR scales to your brightness setting.
On an SDR screen, HLG looks like a normal picture, though BT.2020 colors can look off on a BT.709 screen. On an HDR screen it gets the highlights back. Its color is BT.2020, and it's usually 10-bit, though BT.2100 allows 12.
On a monitor, HLG matters most if you watch live HDR broadcasts through the screen's own apps or edit camera footage shot in HLG. For movies and games, HDR10 and its relatives are far more common.
Verdict
HLG is the practical choice for live TV and some cameras, and it's free to use. On a monitor it's a nice extra rather than a deciding feature. What decides how HLG looks is the same as for any HDR format: how bright the screen gets and how dark it stays.
More HDR Label Decoder scenarios
Related guides
Frequently asked questions
Is HLG better than HDR10?
Does HLG work on an SDR TV?
Who made HLG?
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